Title Atomic defects in titanium dioxide.

Author(s) Minato, Taketoshi

Citation Chemical record (2014), 14(5): 923-934

Issue Date 2014-08-29

URL http://hdl.handle.net/2433/201511

Right

This is the peer reviewed version of the following article:Minato, T. (2014), Atomic Defects in Titanium Dioxide. Chem.Rec., 14: 923‒934, which has been published in final form athttp://dx.doi.org/10.1002/tcr.201402038. This article may beused for non-commercial purposes in accordance with WileyTerms and Conditions for Self-Archiving.; This is not thepublished version. Please cite only the published version. この論文は出版社版でありません。引用の際には出版社版をご確認ご利用ください。

Type Journal Article

Textversion author

Kyoto University

1

Atomic Defects in Titanium Dioxide

Taketoshi Minato

Office of Society-Academia Collaboration for Innovation,

Kyoto University, Gokasho, Uji,Kyoto 611-0011 (Japan)

E-mail: minato.taketoshi.5x@kyoto-u.ac.jp

2

ABSTRACT: The functionality of solid materials is defined by the type and ordering of

the constituent atoms. By introducing defects that perturb the ordered structure, new

functionality is created within the solid material. Atomic defects in titanium dioxide,

such as oxygen vacancies, atomic hydrogen, and interstitial Ti, typically create new

functionality. However, the fundamental physical properties of atomic defects in TiO2

are not fully understood and still remain controversial. In this account, the progress and

issues for debate regarding the physical properties, electronic structure, and

manipulation mechanisms of atomic defects in TiO2 as well as their interaction with

gold nanoclusters are described.

Keywords: defects, electronic structure, solid-state structures, surface analysis, titanium

dioxide

3

1. Introduction

The functionality of solid materials is determined by the type and ordering

structure of their constituent atoms. The introduction of perturbations in the structure of

a material often drastically changes the functionality of the material. For instance,

atomic defects on the surface of titanium dioxide (TiO2) result in changes in its

functionality. TiO2 is used widely for catalysis, photocatalysis, the conduction of

electrons/holes/ions, as a sensing material in photoelectrolysis, and for inducing

photoinduced superhydrophilicity. Furthermore, TiO2 is also used as a pigment, a

cosmetic, a food additive, a filler in pills, a scattering agent for ultraviolet (UV) light,

and a sterilizer. It has been reported that atomic defects such as oxygen vacancies,

atomic hydrogen (hydroxyl), and interstitial Ti have a marked effect on the functionality

of TiO2. However, the mechanisms underlying the changes in this functionality are not

yet fully understood. Thus, clarification of the fundamental physical properties of

atomic defects is necessary to understand these mechanisms.

In this account, I will describe the current knowledge and issues of debate on the

physical properties of atomic defects in TiO2. The electronic structure, the manipulation

mechanisms of atomic defects, and the interaction with gold nanoclusters are selected as

the topics in this account because they are fundamentally important and controversial

topics. Investigations based on surface science techniques such as scanning tunneling

microscopy (STM) and photoelectron spectroscopy (PES) performed in ultrahigh

vacuum (UHV) are mainly discussed this account.

The increase in the number of studies related to atomic defects in TiO2 is

evidence of the interest in the subject. The black curve in Figure 1 indicates the number

of publications N(TiO2 and defect), including research articles, letters, and reviews,

4

corresponding to the keywords “TiO2” and “defect”. This number, which was calculated

using a system provided by the Web of Science™ platform,[1]

reflects the number of

publications since 1959. As observed from the curve, the number of publications was

not high up to 1990, and only five publications were published in 1990. Starting in 1991,

the number of publications has increased gradually. In 2013, the number of publications

was 73.4 times (367 publications) higher than that in 1990. To determine the effect of

this increase with respect to the total number of publications in all journals, we

calculated the following value:

N(TiO2 and defect)

N(total)

where N(total) is the total number of publications in all journals, as determined using

our search system of the Web of Science™ platform. The blue curve in Figure 1 is a

plot of the aforementioned value for different years. It can be seen that this value has

also increased significantly since 1991. This clearly indicates that research interest in

atomic defects on TiO2 surfaces is increasing. The elucidation of the physical properties

of TiO2 using surface science techniques most likely also contributed to this increase in

interest. Previous knowledge regarding the physical properties of atomic defects in TiO2

has been summarized in multiple reviews (Table 1).[2–15]

This account describes the

recent progress and issues of debate for elucidating the physical properties of the atomic

defects in TiO2.

5

2. Structure of Atomic Defects in TiO2

Figure 2 shows the structures of the following atomic defects on the rutile

TiO2(110) surface, which is the most widely investigated TiO2 surface type: a bridging

oxygen vacancy (Obvac), atomic hydrogen (Ha), and interstitial Ti (Tiint). Historically,

Obvac has been believed to be the primary defect and related to the functionality of this

TiO2 surface. Wendt et al. and Bikondoa et al. reported that Obvac reacts readily with

residual water, even under UHV conditions.[16,17]

The reaction produces two Ha defects

on the surface.[16–18]

In reactions involving photocatalysis, photoelectrolysis, and

photoinduced superhydrophilicity, all Obvac vacancies would be filled with water.

Furthermore, it has been recently reported that Tiint exists at the subsurface and plays an

important role in functionality on the TiO2 surface. Previously, Aono and Yagi et al.

suggested that Tiint exists in the bulk of rutile TiO2 and affects electron conduction.[19,20]

Furthermore, Henderson suggested that Tiint is the key point defect in reduced TiO2.[21]

In 2008, Wendt et al. reported that subsurface Tiint in rutile TiO2 can interact with

adsorbates on the surface.[22]

Currently, it is generally believed that Tiint has an

important role in the functionality on the TiO2 surface as well as Obvac or Ha.[23–46]

Oxygen vacancies and atomic hydrogen tend to exist on the surface rather than

in the bulk of rutile TiO2(110). STM images obtained under positive sample biasing

conditions show that Obvac and Ha exhibit similar protrusions towards the row of the

bridging oxygen (Ob) atoms (Figure 3).[16,17,47–51]

In addition, frequency-modulated

atomic force microscopy (FM-AFM) has shown that Obvac and Ha occur as depressions

in the Ob row.[52,53]

The protrusions in the Ob rows observed in STM images obtained

under a positive sample bias were assigned to Obvac[54]

because it is generally believed

that Obvac exists on the surface. Later, it was found that residual water reacts readily

6

with Obvac,[16,17]

and Obvac is filled by the products of water dissociation in many cases.

Furthermore, it was revealed that Obvac and Ha in STM images under positive sample

biasing conditions are similar.[16,17]

In some cases, the assignment of the protrusions in

the Ob rows in the STM images under positive sample bias is changed to Ha.[55]

It is now

possible to distinguish Obvac and Ha in STM images obtained under a positive sample

bias from their apparent heights (that of Ha is slightly greater) and responses to high-

bias-voltage stimulation

(Ha undergoes desorption, and Obvac undergoes lateral diffusion).[16,17,47–51]

The direct observation of Tiint by STM and FM-AFM is not possible because it

exists at the subsurface. For STM imaging of Tiint, the adsorption of oxygen molecules

on the surface allows Tiint to diffuse to the surface. Tiint reacts with the oxygen

molecules, and this can be observed from the higher protrusions in the STM images

(Figure 4).[22]

By using transmission electron microscopy (TEM), Shibata et al.[23]

and

Tanaka et al.[45]

directly observed Tiint.

7

3. Electronic Structures of Defects

The functionality of TiO2 is affected by the introduction of defects. The

electronic structures of the defects on TiO2 surfaces have been investigated extensively.

TiO2 has a band gap of 3.1 eV, and the introduction of atomic defects modifies the

electronic structure in the band gap, forming band-gap states (BGSs). Although

numerous studies have been performed on BGSs, their true nature is not yet fully

understood. In particular, the spatial distribution and origin of the BGSs have been

widely investigated recently; however, they are controversial. In this section, the

controversy regarding the electronic structures of the BGSs induced by atomic defects is

described.

3.1. Spatial Distribution of Band-Gap States

I begin by discussing the BGSs induced by Obvac. The removal of a single

neutral oxygen atom from TiO2 results in two excess electrons in the material. These

electrons occupy the energetically lowest state, mainly formed by the Ti 3d state.

The occupation of the Ti 3d state creates a BGS. The spatial distribution of the BGSs is

of fundamental importance; however, it is still controversial.

In a study based on PES, Henrich et al. showed that the BGSs were created at

approximately 1 eV below the Fermi level in reduced rutile TiO2(110).[56]

X-ray

photoelectron spectroscopy (XPS) has shown that Ti3+

exists on the surface of TiO2,

suggesting the localization of the excess charge at Ti sites.[57]

On the basis of these

results, it was proposed that a BGS is localized at Ti6c under Obvac. Later, numerous

studies were reported on the localization of defect states. A study based on

angleresolved PES (ARPES) (Figure 5) showed that the BGSs were not dispersed.[58]

8

Furthermore, the results of theoretical calculations of the electronic structure of Obvac

suggested that the BGSs were localized at the Ti sites.[59–61]

However, several other

reports have stated that the BGSs are delocalized in TiO2.[62–64]

Henderson et al.

discussed the effects of the delocalization of an extra electron around Obvac on oxygen

dissociation.[65]

By using STM and scanning tunneling spectroscopy (STS), we directly observed

the spatial distribution of BGSs at the atomic scale.[47]

Previously, STM observation of

TiO2 reported on positive sample bias conditions because TiO2 becomes an n-type

semiconductor, and the unoccupied states are close to the Fermi level after TiO2 is

cleaned by annealing and ion sputtering. Via the detection of a small density of the

occupied states, we successfully observed occupied-state images of TiO2. Figure 6

shows the simultaneously obtained images of the unoccupied (Vsample = +0.6 V) and

occupied (Vsample = -1.1 V) states at Obvac sites at 78 K. The STM images clearly show

that the BGSs are not atomically localized; however, they are distributed around Ti5c.

The STS spectra of Ti5c also showed BGSs at approximately -0.9 V (Figure 7).

From these direct observations, we proposed that the BGSs are not atomically localized

but distributed on several Ti5c sites. Using resonant photoelectron diffraction analysis,

Krüger et al. found that extra electrons are present in a wide distribution around the Ti5c

ions, which surround the Obvac sites.[66]

On the other hand, it has been suggested that

BGSs are localized around Obvac by using STM current imaging tunneling spectroscopy

(CITS)-based mapping at liquid helium temperatures.[67]

Furthermore, studies based on

the theoretical calculations of the electronic structure of Obvac have also reported that the

BGSs are localized.[68]

It has been proposed that a delocalized feature is observed when

using STM due to the temperature during observation, because the polaron mobility is

9

high at this temperature.[69,70]

On the other hand, the recent results of density functional

theory (DFT) calculations performed by Cai et al. suggest that the defect states have a

wide distribution in TiO2.[71]

The situation for the investigation of the electronic structure induced by Ha is

similar to that of Obvac. A H atom on Ob gives electrons to TiO2 with local distortion.

The transferred electrons occupy the lowest energy states of TiO2 that are mainly

formed by the Ti 3d states of Ti5c.[61]

We have observed the spatial distribution of BGSs

induced by Ha by occupied state STM at 78 K and proposed that the BGSs are not

atomically localized and distributed on the Ti5c sites as well as Obvac (Figure 8).[47] The

weaker occupation of electrons at the Ti 3d states than at Obvac was also observed for

Ha.[47]

On the other hand, Papageorgiou et al. suggested that the BGSs induced by Ha

are localized on the basis of CITS mapping data at liquid helium temperatures.[67]

The spatial distribution of the BGSs induced by Tiint is still unclear. This is due

to a lack of direct observation of the electronic structure induced by Tiint at the

subsurface by microscopic techniques. DFT calculations showed that the spatial

distribution of the BGSs induced by Tiint is widely distributed.[24,46]

Thus, as described above, the nature of the spatial distribution of the BGSs

remains a topic of debate, and further investigations are necessary to end this

controversy. It is generally accepted that the use of hybrid functionals or GGA+U in

theoretical calculations produces BGSs that are spatially localized.[47,61,69,70]

In contrast,

the use of non-hybrid functionals has shown that they are spatially delocalized.[47,61,63]

However, the distribution of the defect states in the bulk or subsurface regions can be

experimentally determined by direct observation using microscopy techniques. The

development of experimental techniques that allow for the direct observation of the

10

electronic structure of defects including those in the bulk and subsurface regions would

be necessary for resolving the debate surrounding this issue.

3.2. Origin of the Band-Gap States

For a long time, it was believed that oxygen vacancies played an important role

in the formation of BGSs. However, recent reports regarding the BGSs suggested that

oxygen vacancies and other defects both contribute to the BGSs.

In an early study based on PES results, Henrich et al. showed that the BGSs

were created at approximately 1 eV below the Fermi level in reduced rutile TiO2(110)

by annealing in UHV or Ar+ sputtering.

[56] Because Obvac was believed to be the main

defect in rutile TiO2(110), the origin of the BGSs was assigned to the contribution of

Obvac. The assignment of the BGSs to Obvac was believed to be correct and was used for

the interpretation of the functionality of TiO2. The STM work by Wendt et al. and

Bikondoa et al. clarified that Obvac does not exist in many previous experiments.[16,17]

Therefore, the origin of the BGSs had to be reconsidered. In 2006, Di Valentin et al.

showed that both Obvac and Ha create BGSs by DFT calculations.[61]

In 2008,Wendt et al. reported that Tiint has contributions to the BGSs on the

basis of STM, PES, and DFT calculations.[22]

DFT calculations of the electronic

structure of Tiint performed by other groups also showed that Tiint creates BGSs.[24,29,46]

These works suggested that the Obvac, Ha, and Tiint defects have to be considered for the

contribution to the BGSs. In fact, Wendt’s conclusion was that Tiint is almost the only

source of BGSs.[22]

This is entirely in contrast to the previously believed model that

states that Obvac and Ha are the main sources of BGSs. However, the contribution of Tiint

to the BGSs is still controversial, and recent reports have shown that the contribution of

11

Tiint to the BGSs is not as large as Obvac or Ha. For example, our STM/STS results

(Figures 6–8) showed that Obvac and Ha are the sources of BGSs, even though they do

not exclude the contribution of Tiint.[47]

This is not in agreement with the conclusion that

Tiint is the only source of BGSs. Direct evidence of the small contribution of Tiint to the

BGSs was reported by Yim et al. They showed a linear relationship between the density

of Obvac and the intensity of the BGSs via the combined use of STM and PES (Figure

9).[72]

Interestingly, it was concluded that a small portion of the BGSs is attributable to

other defects such as Tiint by extrapolating the experimental results. Furthermore,

Mitsuhara[73]

and Walle[74]

also showed that both Obvac and Tiint are related to the BGSs.

The Obvac sites contribute more to the formation of BGSs than Tiint.[73]

Moreover, Mao

et al. showed that Ha has a major contribution to the BGSs as well as Obvac on the basis

of PES and DFT results, and the contribution of Tiint is smaller than Ha.[46]

As described above, the origin of the BGSs is still not fully understood. The

three types of defects, Obvac, Ha, and Tiint, have contributions to the BGSs,[75]

although

the relative contribution is not the same. In order to accurately evaluate the contribution

of each atomic defect to the BGSs, further conclusive studies are required. For example,

the contribution of each defect could be determined by a comparison of the intensity of

the BGSs with three TiO2 samples each having the same density of one of the three

defects. In addition, direct observation of the electronic structure induced by Tiint would

be ideal to determine the contribution of Tiint to the BGSs; however, this has not been

achieved yet because the observation of the electronic structure induced by Tiint at the

subsurface is not possible by conventional microscopy techniques such as STM. The

development of experimental techniques to directly observe the electronic structure in

the subsurface or bulk would be required.

12

4. Manipulation of Defects

The physical properties of TiO2 are strongly affected by atomic defects. The

control of the densities and surface alignments of atomic defects would result in new

functionality of TiO2. The introduction of defects was carried out by annealing in

vacuum; this induced oxygen molecules to desorb from the surface and produce an

oxygen vacancy.[56]

Furthermore, Ar+ sputtering was used to reduce TiO2.

[56] In 1978,

Knotek and Feibelman reported the photo- and electron-stimulated desorption of oxygen

ions via an interatomic Auger decay process.[76]

The Knotek–Feibelman (K–F) process

is a typical process for inducing ionic desorption via photon and electron excitation.

Zhang et al. reported the effects of the spreading of defect electrons on the desorption of

oxygen from a TiO2 surface.[77]

Recently, the control of the alignment of defects at the atomic scale (i.e., their

manipulation) was achieved using STM. The methods for manipulating

atoms/molecules using STM can be separated into two categories: electron-excitation

processes and non-electron-excitation processes. In electronexcitation reactions, three

excitation mechanisms have been proposed. Tunneling electrons excite electronic states

related to chemical bonds (electronic-state excitation).[78]

The electrons could also be

excited into electronic states related to vibrational states, including translational,

rotational, and conformational change states (vibrational-state excitation) to overcome

reaction barriers by exciting reaction coordinate modes.[79–81]

Furthermore, heating by

electrons also caused reactions owing to electron excitation (manipulation by local

heating).[82]

In nonelectron-excitation reactions, direct interactions between the STM tip

and the atoms/molecules due to the van der Waals/chemical forces (direct

manipulation)[83]

or an electric field to reduce the height of the reaction barrier (electric-

13

field excitation) are exploited to manipulate atoms/molecules.[84–86]

Even though the

underlying mechanisms are not always known, these techniques can be used to control

the alignment of atomic defects on TiO2 surfaces.

The manipulation of defects on rutile TiO2(110) surfaces was first reported by

Diebold et al. in 1998.[87]

They observed the disappearance of defects when scanning at

Vsample = +3.0 V and attributed this phenomenon to the movement of oxygen atoms

between the tip and the Obvac sites on the surface. Later, Suzuki et al. clarified that this

movement was due to the desorption of Ha on the surface (Figure 10).[49]

The

mechanism of the desorption of Ha was studied by Acharya et al., and they proposed

that this reaction was induced by the vibrational excitation of the O–H stretching

mode.[88]

The manipulation of defects on rutile TiO2(110) surfaces using an STM tip to

allow for the in-plane diffusion of Obvac has also been reported. For instance, Diebold et

al. reported the diffusion of Obvac on a TiO2 surface.[87]

Cui et al. reported the diffusion

of a single Obvac induced by excitation from an STM tip.[89]

Using this method, they

were able to form pairs of Obvac; such pairs are rarely observed on surfaces prepared by

cycles of Ar+ sputtering and subsequent annealing. The diffusion of these pairs of Obvac

has also been reported.[89]

The excitation mechanism of this reaction is not known;

however, the application of a high bias voltage is necessary for the reaction to take place.

This suggests that the electric field between the tip and the surface plays an important

role in the reaction.

In addition, we reported that oxygen desorption (i.e., the creation of Obvac) could

occur by applying a negative sample bias from the STM tip (Figure 11).[50]

This reaction

is only possible when the applied sample bias is negative. This result also suggests that

14

the ability to manipulate the defects on a TiO2 surface is related to the electric field

between the tip and the sample surface (Figure 12).[50]

TiO2 is composed of titanium

and oxygen (and adsorbed hydrogen) ions. The modification of the electronic structure

of these ions by an applied electric field plays a key role in the excitation process that

results in the manipulation reactions.

In contrast, the introduction of oxygen vacancies to the anatase TiO2(101)

surface can be achieved by applying a positive sample bias, which induces the migration

of oxygen vacancies from the subsurface region to the surface.[90]

It has been suggested

that the applied electric field has an effect on the migration of oxygen vacancies. The

mechanism underlying the differences between rutile TiO2(110) and anatase TiO2(101)

is still unclear.

15

5. Interaction of Defects with Gold Nanoclusters

TiO2 has been widely used as a support for metal catalysts. One of the early uses

of TiO2 was to disperse expensive precious metals over a high surface area (small

particle size). Later, it was found that the electronic interaction between TiO2 and the

metals influenced the catalytic behavior.[2–15,91]

In particular, the BGSs, which are

located near the Fermi level, strongly interact with the metal; thus, they are considered

to have an important role in the support–metal interaction. Recently, the effects of the

BGSs on the catalytic activity have attracted much attention for the activation process of

Au nanoclusters. Au is a chemically stable metal; however, Haruta et al. discovered that

Au nanoclusters (2–3 nm) supported on TiO2 (or other supports) exhibited a

significantly high activity for CO oxidation at room temperature.[92,93]

Among the

numerous reports on the activation mechanism of Au nanoclusters, the interaction of the

BGSs is believed to have an important role.[93,94]

In this section, the recent progress

related to the roles of the BGSs for the activation of Au nanoclusters is described.

One of the most important issues of the interaction between BGSs and Au

nanoclusters is the electron transfer to change the charge state of Au. Early studies on

work function measurements of Au/TiO2 showed that electrons are transferred from the

supported Au nanoclusters to TiO2; thus, the Au nanoclusters are positively charged on

TiO2.[95]

In contrast, DFT calculations for Au adsorbed at Obvac on TiO2 showed electron

transfer from TiO2 to Au.[63]

On the other hand, DFT calculations by Wahlstrom et al.

showed that the electron transfer is not large between Au and TiO2 with Obvac.[96]

Therefore, the charge state of a Au nanocluster supported on TiO2 has not been

elucidated.

16

We performed PES on reduced TiO2 with different Au coverages and found

that there is a clear shift in the energy of the O 2p states (Figure 13) and a decrease in

the BGSs (Figure 14).[97]

The change in the spectra according to the amount of Au

deposited is attributed to the electron transfer from TiO2 to the Au nanoclusters. This

clearly showed that the Au nanoclusters supported on reduced TiO2 are negatively

charged. Later, the negative charge of the Au nanoclusters on reduced TiO2 was

confirmed via vibrational spectroscopic study of CO molecules adsorbed onto

Au/TiO2.[98,99]

The facilitation of CO oxidation on negatively charged Au nanoclusters

was concluded by the DFT calculations of several groups.[100–104]

In addition, Kelvin

probe force microscopy (KPFM) observation of the charge state of the Au nanoclusters

on reduced TiO2 directly revealed that the Au nanoclusters are negatively charged

(Figure 15).[105]

Therefore, the Au nanoclusters supported on reduced TiO2 are

negatively charged, which would be expected to have an important role in the activation

mechanism for CO oxidation.

The effects of Obvac and Ha on the charge states of Au nanoclusters supported on

reduced TiO2 have been widely studied. More recently, the effects of Tiint on the charge

state of Au nanoclusters on reduced TiO2 have also been investigated. A TEM study

showed that Tiint is found at the interface between Au and TiO2 with a high density,

suggesting that the effects of Tiint on the charge states of the Au nanoclusters should be

considered.[45]

DFT calculations showed that Au adsorbed on TiO2 with Tiint as well as

Au on TiO2 with Obvac and Ha are negatively charged.[24]

In contrast, Lira et al.

suggested that the electron transfer between the Au nanocluster and the reduced TiO2

containing Tiint is not large on the basis of PES measurements.[106]

Further experimental

17

investigations are necessary to confirm the effects of Tiint on the charge state of

supported Au nanoclusters.

It should be noted that the charge states of the supported Au nanoclusters on

TiO2 are significantly affected by the reduction state (amount of defects) of TiO2. PES

and DFT studies showed that Au nanoclusters supported on oxidized TiO2 have a

positive charge.[107,108]

From work on powdered catalysts, positively charged Au

(cationic Au) is thought to have a crucial role in the reaction mechanism of CO

oxidation on Au nanoclusters supported on TiO2.[109]

Positively charged Au would be

produced by changing the reduction states of TiO2.

18

6. Outlook

In this account, the current knowledge and issues of debate concerning the

electronic structure and manipulation mechanisms of the atomic defects in TiO2, as well

as their interaction with Au nanoclusters, are described. Regarding the electronic

structure, the spatial distribution and origin of the BGSs are still under debate. In

particular, information regarding the true nature of Tiint is lacking. As described above,

the direct observation of the electronic structure in the subsurface or bulk region would

provide decisive conclusions. The manipulation mechanisms of the atomic defects of

TiO2 have not been well investigated. Elucidating the mechanisms of the underlying

reactions would allow for alignment control to create new functionality for the metal

oxides. Several investigations on the manipulation of adsorbed atoms or molecules have

been reported.[110–112]

Therefore, the manipulation mechanisms of atomic defects may be

elucidated in the near future. Regarding the interaction of atomic defects with Au

nanoclusters, I personally believe that it has been confirmed that Au supported on

reduced TiO2 is negatively charged. The role of the charge state of Au nanoclusters

under catalytic conditions has been investigated; however, it remains controversial.[94]

Thus, more investigation is required to determine the role of the charge state of Au

nanoclusters in a catalytic reaction.

The atomic defects on rutile TiO2(110) are mainly discussed in this account;

however, the atomic defects on the other facets of rutile TiO2 or anatase TiO2 should be

clarified in the future. Recently, the nature of such atomic defects has been reported. For

example, BGSs have been observed on the (011)-(2×1)[113]

and (100)[114]

surfaces of

rutile TiO2 as well as the (110) surface, although the detailed electronic structures were

not investigated. Furthermore, the BGSs of the (101) and (001) surfaces of anatase TiO2

19

have been observed at approximately 1.0 eV.[115,116]

Interestingly, Moser et al. observed

dispersive behavior of the defect states at 0.1 eV on anatase (001) that is different from

rutile using ARPES,[117]

suggesting that the behaviors of the BGSs on the anatase and

rutile TiO2 surfaces are different. The characteristic nature of the atomic defects on the

other facets of rutile TiO2 and anatase TiO2 need to be investigated.

In addition to the defects that are currently known and investigated, such as

oxygen vacancies, atomic hydrogen, and interstitial Ti, it is expected that new types of

atomic defects will be discovered. If the introduction of metal vacancies or interstitial

oxygen could be controlled, it would lead to new functionalities. The oxygen vacancies,

atomic hydrogen and interstitial Ti in TiO2 confer n-type semiconducting properties by

producing excess electrons. The introduction of metal vacancies or interstitial oxygen

would result in holes in TiO2, leading to novel characteristics.

Finally, clarification of the physical properties of atomic defects in TiO2 is

needed for applications related to chemical reactions and device performance. With

respect to the functionality of TiO2, it should be possible to control the reaction sites for

catalytic and photocatalytic reactions by controlling the distribution of electrons and

holes via defect alignment. Furthermore, defect alignment can also be used to control

the routes for molecular diffusion and the transfer of electrons/holes/ions to the surface.

A recent study reported the effects of defect alignment on the diffusion of alcohol and

water.[118,119]

The techniques used in these instances should find application in

molecular and electronic devices such as batteries and fuel cells as well. To apply the

knowledge obtained from surface science works, the behavior of the atomic defects in

ambient or liquid conditions will be important. The recent developments in

experimental techniques and theoretical calculations elucidate new aspects of TiO2 that

20

could not be observed under UHV conditions.[120–127]

Surface science studies in ambient

or liquid conditions will demonstrate the true nature of atomic defects for application

conditions.

21

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Table 1. Reviews and textbooks related to atomic defects in TiO2.

Authors Journal or book title Year Vol. Page Ref.

number

V. E. Henrich and P. A. Cox The surface science of

metal oxides 1994 [2]

A. L. Linsebigler, G. Q. Lu and J.

T. Yates Chem. Rev. 1995 95 735–758 [3]

U. Diebold Surf. Sci. Rep. 2003 48 53–229 [4]

T. L. Thompson and J. T. Yates Top. Catal. 2005 35 197–210 [5]

T. L. Thompson and J. T. Yates Chem. Rev. 2006 106 4428–

4453 [6]

J. Zhao, B. Li, K. Onda, M. Feng,

and H. Petek Chem. Rev. 2006 106

4402–

4427 [7]

C. L. Pang, R. Lindsay and G.

Thornton Chem. Soc. Rev. 2008 37

2328–

2353 [8]

Z. Dohnalek, I. Lyubinetsky and R.

Rousseau Prog. Surf. Sci. 2010 85 161–205 [9]

M. A. Henderson Surf. Sci. Rep. 2011 66 185–297 [10]

S. Shaikhutdinov, H.-J. Freund Annu. Rev. Phys. Chem. 2012 63 619–633 [11]

A. G. Thomas and K. L. Syres Chem. Soc. Rev. 2012 41 4207–

4217 [12]

Z. Zhang and J. T. Yates Chem. Rev. 2012 112 5520–

5551 [13]

M. A. Henderson and I.

Lyubinetsky Chem. Rev. 2013 113

4428–

4455 [14]

C. L. Pang, R. Lindsay and G.

Thornton Chem. Rev. 2013 113

3887–

3948 [15]

31

Fig 1. The black [N(TiO2 and defect)] and blue [N(TiO2 and defect)/N(total)] curves

were determined using the Web of Science

platform.[1]

Here, N(TiO2 and defect) and

N(total) are the number of publications corresponding to the keywords “TiO2” and

“defect” and the total number of publications in all journals, respectively. Articles,

letters, and reviews were included in these calculations.

350

300

250

200

150

100

50

0

Num

ber

of

pub

licatio

ns

(TiO

2 a

nd

de

fect)

201020001990198019701960

Year

2.0

1.5

1.0

0.5

0.0

Num

ber o

f public

atio

ns (T

iO2 a

nd

de

fect)/

tota

l num

ber o

f pub

licatio

ns / 1

0-3

N(T

iO2

an

d d

efe

ct)

N(T

iO2

an

d d

efe

ct)/N

(tota

l) / 10

-3

32

Fig. 2. Surface structure of the bridging oxygen vacancies (Obvac), atomic hydrogen (Ha),

and interstitial Ti (Tiint) on the rutile TiO2(110) surface. The bridging oxygen (Ob), five-

coordinated Ti (Ti5c), and six-coordinated Ti (Ti6c) are also shown. The red and gray

balls represent oxygen and titanium ions, respectively.

Tiint

HaOb

Obvac

Ti5c

Ti6c

33

Fig. 3. 6.5 × 2.6 nm2 constant-current STM images of Ha (OH in figure) and Obvac on a

rutile TiO2(110) surface. Vsample = +1.5 V and Itunnel = 0.3 nA at 78 K. (a) Before and

(b) after scanning at Vsample = +3.0 V and Itunnel = 0.3 nA.[47]

Copyright 2009 American

Institute of Physics.

34

Fig. 4. 15 × 15 nm2 STM image of a rutile TiO2(110) surface with TiOx formed by a

reaction between Tiint and oxygen molecules. Reprinted with permission from reference

[22]. Copyright 2008 American Association for the Advancement of Science.

35

Fig. 5. ARPES spectra of a reduced TiO2(110) surface obtained along the off-normal to

the [001] direction.[58]

Copyright 1994 Elsevier.

36

Fig. 6. 3.4 × 5.8 nm2 constant-current STM images of Obvac vacancies on a rutile

TiO2(110) surface at 78 K. (a) Unoccupied-state image (Vsample = +0.6 V and Itunnel =

0.6 nA) observed in the forward scanning mode, and (b) occupied state image (Vsample =

-1.1 V and Itunnel = 0.1 nA) observed in the reverse scanning mode. Magnified images

(2.1 × 1.3 nm2) of the area around the Obvac sites [yellow rectangles in (a) and (b)] are

also shown. The open red circles indicate the positions of the Obvac sites.[47]

Copyright

2009 American Institute of Physics.

37

Fig. 7. STS current (I)–voltage (V) (inset) and dI/dV curves of the positiondependent

occupied density of states at an Obvac site. The measurement sites are indicated in the

occupied-state STM image. The set-point values of Vsample and Itunnel were fixed at

+1.5 V and 1.0 nA, respectively.s Copyright 2009 American Institute of Physics.

38

Fig. 8. (a) An unoccupied-state image at Vsample = +1.4 V and Itunnel = 0.01 nA and (b)

an occupied-state image at Vsample = -1.4 V and Itunnel = 0.005 nA for Obvac and Ha on

rutile TiO2(110) (image size: 5.8 × 6.6 nm2, Tsample = 78 K). High-contrast images of

(c) Ha and (d) Obvac.[47]

Copyright 2009 American Institute of Physics.

39

Fig. 9. Normalized integrated intensities of the band-gap states of ultraviolet

photoelectron spectroscopy (UPS) peaks as functions of the Obvac density on a rutile

TiO2(110) surface, determined using STM. (a) Data taken from experiments performed

on TiO2 (color of the plots indicates the different densities of the Obvac. The details are

described in ref. [72]). (b) Data taken from experiments performed on hydroxylated

TiO2(110). The data points are for the initially hydroxylated surface (filled square), the

oxidized surface (filled circle), and the oxidized surface subjected to electron

bombardment for 2 (open square), 5 (open circle), and 10 s (open triangle).[72]

Reprinted

with permission from reference [72]. Copyright 2010 American Physical Society (http://

journals.aps.org/prl/abstract/10.1103/PhysRevLett.104.036806).

40

Fig. 10. STM-induced desorption of Ha on the rutile TiO2(110) surface. The constant-

current topography of the surface is shown after the high-bias scan for manipulation.

Manipulation scan: 20 × 20 nm2, Vsample = +3.0 V, and Itunnel = 0.30 nA. Imaging

scan: 50 × 50 nm2, Vsample = +2.0 V, and Itunnel = 0.30 nA.

[49] Reprinted with

permission from reference [49]. Copyright 2000 American Physical Society

(http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.84.2156).

41

Fig. 11. Constant-current STM images of a rutile TiO2(110) surface (a) before and (b)

after pulse injection to an Obvac site. (c) A constant-current STM image obtained after

applying a pulse to the species that appears at the center of the STM image in (b). All

STM images were obtained at Vsample = +1.5 V, Itunnel = 0.3 nA, and Tsample = 78 K (3.5

× 2.9 nm2). The applied pulses were Vsample = -2.0 and +4.0 V for 1 s to Ob and the

species that appeared (Obvac), respectively, with the feedback loop turned off.[50]

Reproduced from reference [50] with permission. Copyright 2012 Cambridge

University Press.

42

Fig. 12. A schematic image of the reactions of the atomic defects on a rutile TiO2(110)

surface with an STM tip. The gray, red, and white balls are Ti, O, and H atoms,

respectively.[50]

Reproduced from reference [50] with permission. Copyright 2012

Cambridge University Press.

43

Fig. 13. PES spectra of Au/TiO2(110) as a function of Au coverage.[97]

Copyright

2004 Elsevier.

44

Fig. 14. PES spectra of Au/TiO2(110) near the Fermi level as a function of Au

coverage.[97]

Copyright 2004 Elsevier.

45

Fig. 15. Simultaneously obtained (a) non-contact atomic force microscopy topography

and (b) Kelvin probe force microscopy images of Au adsorbed onto a hydroxylated

rutile TiO2(110) surface (25 × 25 nm2). The (c) height and (d) local contact potential

difference profiles were taken along the lines drawn in (a) and (b), respectively. The

circles indicate Au clusters.[105]

Copyright 2011 American Institute of Physics.